Industrial electrolyses such as caustic alkali electrolysis perform an important role in material industry, but the energy used for such electrolyses is large. Hence, energy efficiency in these electrolyses is a problem.
In caustic alkali electrolysis, an initial mercury method is converted into an ion-exchange method through a diaphragm method resulting in less pollution and energy savings of about 40%. However, the cost for the electric powder accounts for 50% of the total production cost.
For further energy savings, the use of gas diffusion electrodes which have been investigated and developed in the field of cells such as fuel cells has been attempted. When the gas diffusion electrode is applied to an ion-exchange membrane type sodium chloride electrolysis which is the most advanced in energy savings at present, energy savings of more than 50% becomes theoretically possible. Accordingly, various investigations have been made to find ways of practically using gas diffusion electrodes. EQU 2NaCl+2H.sub.2 O.fwdarw.Cl.sub.2 +2NaOH+H.sub.2 E.sub.0 =2.21 volts EQU 2NaCl+1/2O.sub.2 +H.sub.2 O.fwdarw.Cl.sub.2 +2NaOH E.sub.0 =0.96 volt
The structure of the gas diffusion electrode being used for caustic soda electrolysis is of the so-called semihydrophobic (water repellent) type and is characterized by a hydrophilic reactive layer adhered to a hydrophobic gas diffusion layer. Both the reaction layer and the gas diffusion layer use carbon as the main raw material and a PTFE resin as the binder. The PTFE resin is hydrophobic, and by increasing the proportion of the resin in the gas diffusion layer and reducing the proportion of the resin in the reaction layer by utilizing this property, the aforementioned characteristics are exhibited. Furthermore, since in caustic soda electrolysis, the gas diffusion electrode is used in an aqueous caustic alkali solution having a high concentration, the PTFE resin which is a hydrophobic material sometimes becomes hydrophilic. A gas diffusion electrode wherein a thin porous PTFE resin sheet is placed on the gas diffusion layer at the gas chamber side is proposed to prevent this phenomenon.
The surface of the reaction layer carries a catalyst such as platinum, etc., or the catalyst is carried on the surface of carbon constituting the reaction layer. These electrodes are all prepared by using a fluorine resin as the binder, solidifying by heating the fluorine resin together with a carbon powder carrying an electrode substance, and carrying the solidified mixture on a substrate such as titanium, nickel, stainless steel, etc. Production is easy for such electrodes, although such a three-dimensional solid skeleton of forming a strong sheet is not formed as in case of using PTFE, etc. When such a gas diffusion electrode is used as a cathode for carrying out the depolarization of oxygen by sending an oxygen-containing gas, even if the crosslinking of a fluorine resin is insufficient, the gas diffusion electrode can be used under stable operating conditions with a sufficiently satisfactory performance at the beginning of the use since the carried electrode substance can exist stably. However, not only a carbon powder but also a fluorine resin is not always stable in an alkali.
During electrolysis, an oxygen-containing gas is supplied to the gas diffusion electrode, but the oxygen-containing gas forms hydrogen peroxide and the hydrogen peroxide corrodes carbon to form sodium carbonate. Sodium carbonate clogs the gas diffusion layer in an alkali solution, reduces the hydrophobic property of the electrode after extended use and may decrease the activity of the electrode substance. Further, corrosion of the carbon gradually proceeds in only the presence of the catalyst metal even when hydrogen peroxide is not generated.
For solving these problems, the selection and the production method of carbon conventionally used, control of the mixing ratio of the resin and carbon, etc., have been attempted. However, the fundamental problem is not thereby solved in any case, and the progress of the corrosion of the carbon can be delayed but cannot be stopped.
These problems occur as a result of using carbon as a material of a gas diffusion electrode. A gas diffusion electrode using metal having corrosion resistance in place of carbon has been proposed. However, the gas diffusion electrode is produced by a sintering method different from the method of producing a conventional gas diffusion electrode having carbon, and the production method is very complicated. Further, the hydrophilic portion and the hydrophobic portion are difficult to control.
The problems described above are the primary reason why a gas diffusion electrodes have not been widely used in the process of producing caustic soda (sodium hydroxide) and chlorine by sodium chloride electrolysis or producing caustic soda and sulfuric acid by Glauber's electrolysis.